1. Field of the Invention
This invention relates to a disc-shaped recording medium, a disc recording apparatus and a disc reproducing apparatus. Note particularly, it relates to a disc-shaped recording medium, a disc recording apparatus and a disc reproducing apparatus adapted for recording and reproducing data by e.g. optical means.
2. Description of the Related Art
An optical disc may have a recording capacity larger by two or three orders than that of a magnetic disc, while enabling accessing at a higher speed than that for a tape recording medium. An optical disc also has the advantage of contactless data recording and reproduction, so has good durability and has come to be used extensively. A well known form of optical disc is the so-called compact disc (CD).
To provide a portable and, above all a pocketable headphone stereo or similar recording/reproducing apparatus using an optical disc, a CD with a diameter of 12 cm and a CD diameter of 8 cm have been proposed. However, with a disc diameter of 12 cm, the recording/reproducing apparatus is too bulky to be readily portable. Therefore, a disc 8 mm or less in diameter would be preferred. However, in attempting to construct a portable or pocket size recording and/or reproducing apparatus for an optical disc 8 cm or less in diameter, the following problems arise.
In the case of a standard CD format optical disc, on which are recorded stereophonic digital PCM audio signals sampled with a sampling frequency of 44.1 kHz and quantized with 16 bits, and where these signals are exclusively reproduced by the user (CD-DA format), the playback time (recording time) of a disc 8 cm in diameter is 20 to 22 minutes at most, meaning that a symphony, for example, cannot be recorded on one disc side. A playback time of 74 minutes or longer, which is approximately equal to that of a 12 cm CD, is preferred. Besides, with this CD-DA format), recording by the user cannot, be made. In addition, a contact, less type optical disc device is weak to mechanical vibrations and subject to detracking and defocusing. Thus, when the apparatus is to be portable, some positive measures need to be taken to prevent adverse effects of the detracking or defocusing on the reproducing operation.
On the other hand, with a CD-MO format (a format, employing recordable magneto-optical disc) as an extension format of the above mentioned standard CD format (CD-DA format), the recording/reproducing time of an 8 cm disc is only 20 to 22 minutes, as in the CD-DA format. The optical pickup device is similarly susceptible to detracking or defocusing due to mechanical vibrations, such that again positive measures need to be taken to prevent any adverse effects thereof on the recording/reproducing operation.
In the so-called CD-I (CD-Interactive) format or in the so-called CD-RON/XA format, the levels shown in Table 1 are prescribed as the modes for recording/reproducing bit-compressed digital audio signals:
TABLE 1 ______________________________________ SAMPL- QUANTIZ- PLAYBACK ING ED TIME FRE- NUMBER BAND- STEREO/ LEVEL QUENCY OF BITS WIDTH MONAURAL ______________________________________ A 37.8 kHz 8 17 kHz 2/4 B 37.8 kHz 4 17 kHz 4/8 C 18.9 kHz 4 8.5 kHz 8/16 ______________________________________
Referring to Table 1, when reproducing a disc recorded with, for example, the level B mode, signals obtained by 4-fold bit compression of standard CD-DA format digital signals are reproduced. Thus, if all of the recorded data are stereophonic audio compressed data, playback time may be increased 4-fold, or reproduction of 4-channel data becomes possible, so that reproduction for 70 minutes or longer may be made with an optical disc 8 cm in diameter or less.
Meanwhile, with the CD-I format, the disc is rotationally driven at the same linear velocity as that with the standard CD-DA format, so that continuous audio compressed data are reproduced at a rate of one unit to n playback units on the disc, where n is a figure corresponding to the bit compression factor or the above-mentioned playback time and is equal to four in the level B stereo mode. This unit is termed a block or sector, which is made up of 98 frames and has a period of 1/75 second. Therefore, with this level B stereo mode, a data string in which one of four sectors is an audio sector, such as:
SDDDSDDD . . .
where S is an audio sector and D is another audio sector or sectors, is recorded on a sector-by-sector basis on the disc. However, for actual recording, since the above data string undergoes a predetermined encoding similar to that for ordinary CD format audio data, such as error correction coding and interleaving, data of the audio sector S and data of the data sector D are arranged in a scrambled fashion in the recording sectors on the disc. The other data sectors D may, for example, be video or computer data. When the bit-compressed audio signals are also used for the data sector D, a data string in which 4-channel audio sectors S1 to S4 are cyclically arranged; that is a data string:
S1 S2 S3 S4 S1 S2 S3 S4 . . .
encoded and recorded on the disc. When recording and reproducing continuous audio signals, the above-mentioned 4-channel audio signals are linked sequentially beginning at the first channel and terminating at the fourth channel. More specifically, channel 1 data corresponding to the audio sector S1 are reproduced from the innermost to the outermost areas of the disc. Channel 2 data corresponding to the audio sector S2 are reproduced from the innermost to the outermost areas of the disc. Channel 3 data corresponding to the audio sector S3 are reproduced from the innermost to the outermost areas of the disc. Finally, channel 4 data corresponding to the audio sector S4 are reproduced from the innermost to the outermost areas of the disc to enable data reproduction for a continuous 4-fold time duration.
However, for the above mentioned continuous reproduction, several track jump operations spanning the inner and outer disc peripheries are necessitated. Since a track jump cannot be achieved instantaneously, playback data are interrupted for a short time period. This means that the playback sound is momentarily interrupted. On the other hand, when continuous audio signals are to be recorded, it is not possible to record only the sector 2 signals, as an example, because the data need to be interleaved at the time of recording. That is, sector 2 data need to be interleaved with adjoining and even near-by sectors, such as sectors S1 and S3, such that it becomes necessary to rewrite signals of previously recorded sectors. Thus it is extremely difficult to effect realtime recording of these continuous compressed audio signals.
Moreover, it is desirable to permit switching selection between the above-mentioned compression modes, because then the usage of the recording/reproducing apparatus can be significantly increased. However, if it is necessary to change the rotational velocity of the disc, the recording pattern, or the signal processing operation, for each of the selected compression modes, the circuitry becomes complex, while it is difficult to make an inexpensive apparatus. It is therefore desirable that changes brought about in the control operations, recording patterns or signal processing operations as a result of the different compression modes be as small as possible.
For overcoming the above-mentioned problems, the present Assignee has proposed the following technique in our copending U.S. Pat. No. 5,243,588, issued Sep. 7, 1993.
Specifically, this technique resides in arranging digital data into a plurality of clusters at an interval of a predetermined number of, for example, 32, sectors, providing e.g. five cluster-linking sectors at the linking portions of each of the clusters, each of the cluster-linking sectors being longer than an interleaving length for digital data so that data interleaving in one cluster does not affect the adjoining clusters.
Meanwhile, as addresses supervising data on a disc, that is addresses indicating data positions on the disc, the absolute time, such as minutes, seconds and sector numbers shown in FIG. 1, recorded in a subcode Q in the CD-DA format, in a header data in the CD-ROM format and by modulating the guide groove in the CD-MO format, is employed. Such absolute time is also employed for indicating the actual play time.
However, if the digital audio signals are recorded with bit compression, and the compression factor is other than a simple integer, such as two or four, the actual play time can be calculated only by a time-consuming operation. Besides, since the linking sector does not contribute to the actual play time, it is necessary to subtract the linking sectors in calculating the actual play time, so that more time is consumed in calculating the actual play time. Besides, in recording data, it is necessary to begin with the linking sector and calculate to which absolute time this linking sector corresponds.